There have been a number of publications relating to the development of color changing materials. Photonic crystals are peculiar structures that show periodic variations in refractive index on a length scale comparable to the wavelength of light. This periodicity means that, for certain ranges of energies and wave vectors, light is not allowed to propagate through the medium. Such disallowed groups of wavelengths are called photonic band gaps. The coloration thereby imparted to photonic structures is called structural color, since it is not due to the presence of a dye or pigment, but rather to the conformation of the material itself. Photonic crystals are found in nature, e.g. in beetle scales, butterfly wings and parrot feathers, and can also be fabricated using a wide range of techniques.
In recent years, it has been discovered that subjecting a liquid suspension of iron oxide colloidal nanocrystal clusters (“CNCs”) to a magnetic field causes the CNCs to assemble into periodic arrays that form a photonic crystal, which diffracts light in the visible spectrum, as well as in the ultraviolet and infrared spectrums. Adjusting the strength of the magnetic field applied to the CNCs alters the photonic crystal structure and hence the wavelength (color) of the diffracted light. In other words, the color displayed by the CNCs may be controlled by altering the strength of a magnetic field applied to a suspension containing the CNCs.
One of the earliest publications describing the development of particles that have color-changing attributes is WO2009/017525, which describes development of superparamagnetic magnetite (Fe3O4) CNCs. Polyacrylic acid is used as a surfactant for the strong coordination of carboxylate groups with iron cations on the magnetite surface. WO2009/017525 also teaches a method for constructing colloidal photonic crystals out of the polyacrylate capped superparamagnetic magnetite CNCs. The colloidal photonic crystals show highly tunable diffractions covering the whole visible region owing to the highly charged polyacrylate covered surfaces and the strong magnetic responses of the magnetite CNCs. These magnetite CNCs readily self-assemble into colloidal photonic crystals in polar solvents (such as water and alkanols) upon application of a magnetic field. The optical responses of the photonic crystals are rapid and fully reversible.
WO2010/096203 teaches a method of assembling superparamagnetic CNCs into colloidal photonic crystals in nonpolar solvents by establishing long-range electrostatic repulsive forces on the CNCs using charge control agents. The method includes coating the CNCs with a hydrophobic coating so that the CNCs are soluble in a nonpolar solvent solution, and adding a surfactant (charge control agent) to the nonpolar solvent solution, wherein the surfactant enhances charge separation between the CNCs to form an ordered structure with tunable particle separation.
WO2012/051258 describes a method of forming photonic crystals that diffract light to create color by dispersing solid particles within a magnetic liquid media, and magnetically organizing the solid particles within the magnetic liquid media into colloidal photonic crystal structures. The solid particles are non-magnetic, and the magnetic liquid media is magnetic nanoparticle-based ferrofluid, which is prepared by dispersing magnetic nanoparticles of transition metal and metal oxides in a liquid medium. The ferrofluid may be created in a polar or nonpolar solvent.
WO2013/006207 describes a method of producing multifunctional composite particles by direct self-assembly of hydrophobic nanoparticles on host nanostructures containing high density surface thiol groups. Hydrophobic nanoparticles of various compositions and combinations can be directly assembled onto the host surface through the strong coordination interactions between metal cations and thiol groups. The resulting structures can be further overcoated with a layer of normal silica to stabilize the assemblies and render them highly dispersible in water.
WO2010/120361 teaches a method wherein CNCs are coated in shells of other suitable mediums, such as silica, titania (titanium oxide), and/or polymers such as polystyrene and polymethylmethacrylate, in which the coating provides a means to obtain good dispersibility and promote solvation repulsion in a photocurable solution or resin. The coated CNCs are then dispersed in the photocurable solution or resin, after which the photocurable solution or resin containing the CNCs is placed in an immiscible solution (such as an oil) to form an emulsion. The emulsion is exposed to an external magnetic field to align the coated CNCs in one-dimensional chains within emulsion droplets within the photocurable solution or resin, and the emulsion droplets are cured within the photocurable solution or resin into magnetochromatic microspheres so that the color displayed by the CNCs is fixed when the magnetic field is removed. The magnetochromatic composition may be used for a color display, signage, bio and chemical detection and/or magnetic field sensing.
WO2013/112224 teaches a method of stabilizing electromagnetically charged particles, which includes coating electromagnetically charged particles with a protective layer and etching the protective layer with silica to produce a porous protective layer.
WO2012/122216 describes a method of fabricating individually fixed nanochains with a magnetically responsive photonic property, wherein CNCs are coated with a layer of silica, a magnetic field is applied to the CNCs to assemble the CNCs into photonic chains, and the photonic chains are then overcoated with an additional layer of silica. The particle chains are then permanently fixed by the silica overcoating so that they remain stable when dispersed in solution or dried on solid substrates.
WO2012/023991 describes a device for tuning bistable materials, such as a polymer or other media/medium containing CNCs. In certain embodiments, the tuning device transfers energy to the CNCs, which in turn locally softens or melts the thermally reversible polymer immediately surrounding each CNC, thus allowing the CNCs to reorient locally within the polymer when a magnetic field is applied for tuning of the color displayed by the bistable material. In other embodiments, an ionizing radiation (IR) device may be used in place of heat.
WO2011/126575 describes a color changeable artificial nail, in which the nail is formed of a bistable material, such as a cholsteric liquid crystal layer, that is adapted to change color in response to the application of an electrical charge.
While these publications describe how to form colloidal photonic crystals from CNCs suspended in polar and nonpolar liquid solvents, how to magnetically organize non magnetic solid particles within a ferrofluid containing CNCs, how to incorporate CNCs onto a host surface, how to permanently fix the color displayed by the CNCs in a UV curable resin or by application of a silica overcoating, as well as how to reversibly fix the color displayed by the CNCs in a hi stable medium, there still remains a need for a method of incorporating CNCs into or applying CNCs to materials for use in the manufacture of apparel, footwear, sports equipment, and accessories, and fixing the color displayed by the CNCs (reversibly or permanently) so that the color does not change when the magnetic field is removed.
Furthermore, there are limitations in the current technologies available for manufacturing articles with details of a different color than the background. For example, two-tone colors in fabrics are typically accomplished through either sublimation of the color pattern onto a substrate or woven into the base fabric with two different colored yarns. While sublimation may produce vivid colors, it is difficult to reproduce sharp lines with this process when the details are less than 3 mm in thickness, such as the thickness of the chevrons in the jersey shown in FIGS. 1A and 1B. Thus, there is a further need to develop an improved process for manufacturing articles with details of a different color than the background.